There are many U.S. patents and patent applications related to electromagnetic surveying. Some of the more relevant ones appear to be the following: U.S. Pat. No. 6,253,100, for broad band electromagnetic holographic imaging; U.S. Pat. No. 7,203,599, for acquiring transient electromagnetic survey data; U.S. Pat. No. 7,337,064, for electromagnetic surveying for hydrocarbon reservoirs; U.S. Pat. No. 7,483,792, for electromagnetic surveying for hydrocarbon reservoirs; U.S. Pat. No. 7,502,690, for using time-distance characteristics in acquisition of t-CSEM data; U.S. Pat. No. 7,565,245, for electromagnetic surveying; U.S. Pat. No. 7,805,249, for controlled source electromagnetic surveying with multiple transmitters; U.S. Pat. No. 7,822,562, for removing air wave noise from electromagnetic survey data; U.S. Pat. No. 7,941,273, for using time-distance characteristics in acquisition of T-CSEM data; 20080105425, for electromagnetic surveying for hydrocarbon reservoirs; 20090005994, for time lapse analysis with electromagnetic data; 20090005997, for spatial filtering of electromagnetic survey data; 20090067546, for compensating electromagnetic data; 20090072831, for real time monitoring of the waveform transmitted by an electromagnetic survey; 20090082970, for electromagnetic surveying; 20090103395, for wavelet denoising of controlled source electromagnetic survey data; 20090120636, for controlled source electromagnetic surveying with multiple transmitters; 20090126939, for electromagnetic data processing system; 20090204330, for using time-distance characteristics in acquisition of T-CSEM data; 20090265111, for signal processing of marine electromagnetic signals; 20090276189, for estimating noise at one frequency by sampling noise at other frequencies; 20100018719, for inversion of CSEM data with measurement system signature suppression; 20100065266, for controlled source electromagnetic reconnaissance surveying; 20100176791, for correcting the phase of electromagnetic data; 20100224362, for electromagnetic imaging by four dimensional parallel computing; 20100233955, for electromagnetic air-wave suppression by active cancellation; 20110013481, for detecting marine deposits; and 20110087435, for electromagnetic prospecting waveform design. All of these patents and patent applications are incorporated herein by this reference.
Several techniques exist that attempt to remove air wave noise and other noise sources from the signal of interest in a CSEM system. These techniques include active filtering, signal encoding such as grey coding, and noise estimation and subtraction at different frequencies. Additionally, until recently, all of these techniques were supplemented by physical isolation of the receiving elements from the noise source, by submersion in a marine environment, thus using the water as an air wave signal filter. These methods are typified in the above-listed U.S. patent applications 2009/0204330, 2009/0265111, 2009/0276189, 2011/0013481, and in U.S. Pat. No. 7,822,562.
The major problem with these techniques is that they are unable to successfully filter out in-frequency noise because the frequency of interest is very close to the frequency of the noise, that is, typically between fifty and sixty hertz (50-60 hz).
In addition, for the purposes of operation on the surface, there are many more sources of noise and amplification of noise, such as rail lines, pipelines and barbed wire fences, that is, anything that is ferrous and long. The typical solution to these noise problems is to survey the area before performing a CSEM survey, and remove the known anomalies from the data. Items can be missed in the preliminary survey, causing additional unexpected noise in the data, and thus reducing delineation and depth of investigation. A person skilled in the art of performing CSEM surveying will understand the issues that uncontrolled noise can cause when using existing systems for surface based measurements. The use of CSEM for surveying is described in U.S. Pat. No. 7,203,599.
In light of the foregoing, a need remains for a system and method of visualizing sub-surface formations that reduces noise, and improves resolution.